In recent years, white LED devices have been put into use as illumination light sources in place of filament lamps. For such use, there is a demand to provide white LED devices with power saving performance and high color-rendering properties. Many of currently available white LED devices realize pseudo white light by combination of YAG-Ce yellow phosphors and GaN blue LEDs.
However, the conventional combination of YAG-Ce yellow phosphors and blue LEDs faces the problem that cyan component light (up to 500 nm) and red component light (600 nm or more) are not sufficient. It is thus common practice to supplement these insufficient component lights by mixing a plurality of phosphors. For example, Patent Document 1 discloses a high-color-rendering white light source using a YAG-Ce yellow phosphor in combination of a red-emitting Eu complex.
On the other hand, there occur large variations in emission color between white light emitting devices as discussed in Patent Document 2 in the case where the white light emitting devices are produced to achieve high color-rendering properties by mixing a plurality of phosphors as mentioned above. Even when the white light emitting devices are each produced with a plurality of phosphors, it is difficult for the white light emitting devices to achieve high color-rendering properties due to very low emission intensity at wavelengths midway between the center emission wavelengths of the respective phosphors. The conventional combination of YAG-Ce yellow phosphors and blue LEDs also has the problem that the use of a part of blue excitation light as illumination light leads to variations in the color of the illumination light depending on the thickness of the phosphor layer through which the blue light passes (Patent Document 3) even though it is advantageous in terms of a small strokes shift and high emission efficiency by a small difference between excitation wavelength (about 450 nm) and fluorescence wavelength (center wavelength: about 550 nm; wavelength shift depending on the amounts of Gd and Y added). It has been suggested that, in the white light emitting device where the yellow phosphor layer is mounted on the blue LED element, the color of the illumination light varies between the center and periphery of the element.
There are known materials each capable of emitting white light by itself, such as a fluoride glass containing divalent ytterbium ions (Yb2+) (Patent Document 4) and fluorophosphate salt glasses containing blue fluorescent divalent europium ions (Eu2+) and phosphorus (P) (Patent Document 5 and Non-Patent Document 1).
Furthermore, Patent Document 4 proposes a white light emission material that uses: a fluoride glass containing 20 to 45 mol % of AlF3; 40 to 65 mol % alkaline-earth fluorides in total, including 0 to 15 mol % of MgF2, 7 to 25 mol % of CaF2, 0 to 15 mol % of SrF2 and 5 to 25 mol % of BaF2 and 10 to 25 mol % of at least one fluoride of element selected from the group consisting of Y, La, Gd and Lu; and Yb2 incorporated as a luminescent center in the fluoride glass.